This invention relates generally to the desorption of contaminated adsorbent materials using microwave energy and more particularly concerns an improved batch system for effective microwave desorption.
In industry, process streams carrying contaminants or other components are often purified by passing the stream in contact with an adsorbent. The contaminants or other components are adsorbed by the adsorbent, thereby removing them from the process stream. Materials commonly used as adsorbents include activated carbon, activated charcoal, zinc oxide, activated alumina and molecular sieves. Adsorption is most effective when the adsorbent is maintained at ambient temperatures or cooler. The adsorbed materials are referred to as adsorbates or simply sorbates. Thus, a sorbated adsorbent refers to an adsorbent having adsorbed materials therein. In the course of cleansing process streams, the adsorbent will eventually become saturated with sorbates and be unable to adsorb further materials. Rather than simply being disposed of, a saturated adsorbent can be recycled through a process which desorbs or strips the sorbates from the adsorbent. Once the sorbates have been desorbed, the adsorbent is again capable of being used to cleanse process streams.
Many organic contaminants can be desorbed by heating the adsorbent to relatively low temperatures (e.g., in the range of 100.degree.-300 .degree. C. for activated carbon). This low temperature process is referred to as regeneration. However, some contaminants cannot be desorbed at regeneration temperatures. These remnant contaminants, which might be high boiling point materials or result from polymerization on the adsorbent, are referred to as the "heel." After many (hundreds or even thousands) regenerations, the heel buildup diminishes the sorbent capacity of the adsorbent to the extent that the adsorbent is no longer useful. At this point, the adsorbent must not only be treated at higher temperatures (e.g., about 900.degree.-1000 .degree. C. for activated carbon) but must also be exposed to reactants (such as steam or carbon dioxide) which can gasify some of the heel and the adsorbent to create new surface area. This process is called reactivation and is usually performed in large, off site furnaces. As used herein, the terms "desorption" or "desorbing process" refer to both regeneration and reactivation.
It is desirable to employ a desorbing process which is capable of stripping the sorbates on the plant site, because such a process eliminates the need to ship the sorbated adsorbent off site for cleaning. Besides offering cost advantages, on-site desorption reduces the number of plant emissions which must be reported to the Environmental Protection Agency, making additional uses in the plant more economically attractive.
A typical on-site method of regenerating a saturated adsorbent is to heat the adsorbent with a flow of hot gas such as steam, nitrogen or flue gases to a sufficiently high temperature at which the sorbate will be desorbed. The high temperature causes the sorbated matter to vaporize and pass from the adsorbent. The flow of the hot gas also purges the vaporized or desorbed materials from the system. The adsorbent bed must be taken off-line to be swept with the hot gas. Some disadvantages of this gas heating method include long regeneration times, large amounts of purge gas, non-uniform heating of the adsorbent material, and dilution of the sorbate vapors with heating gases. This dilution is a problem when the desorbed materials must subsequently be removed from the purge gases. Furthermore, the gas heating method requires heating not only the adsorbent material but also the entire structure containing the adsorbent, which is necessarily several times heavier than the adsorbent. Thus, the design of the containment structure is controlled by the temperature and corrosion limits prescribed by the regeneration process. In addition, this type of gas heating usually can achieve temperatures only in the range of about 100.degree. -150 .degree. C. and is thus insufficient for reactivation.
Microwave heating of the adsorbents has been proposed to avoid some of the problems associated with the hot gas heating method. Microwave heating has an advantage in that the adsorbent material can be heated without directly heating the containment structure. Thus, the energy required for microwave heating is less than heating with hot gas. The cost of the containment structure can also be reduced since the structure itself is subjected to lower temperature ranges. Microwave heating also can produce generally higher temperatures than the gas heating method. Therefore, microwave heating is readily applicable to reactivation, as well as regeneration.
One microwave desorption approach is set forth in U.S. Pat. No. 4,737,610 to Kotsch et al. The Kotsch et al patent discloses a method and apparatus using a gravity-driven moving bed for the desorption of noxious materials from a carbonaceous adsorption agent. Saturated carbon or coke is fed into the regeneration unit via a dosing and closure unit 1. The coke falls into a quartz conduit 2 where it is heated by a microwave heating means. The coke then enters a desorption gas collector 6 where it builds up a free fill above a perforated conical plate 7. In the desorption gas collector 6, the coke is swept with an inert gas to apparently remove the desorbed noxious materials. In a second embodiment, the quartz conduit is replaced with a horizontal moving belt which conveys coke through a heating chamber prior to dumping the coke into the desorption gas collector 6 by gravity feed. While the Kotsch et al patent solves many of the problems associated with hot gas heating, other drawbacks arise. For instance, there is a high degree of relative movement between individual coke granules as well as between the coke granules and the walls of the container due to the continuous, free fall method of moving the coke. This relative movement tends to grind some of the coke into smaller, less useful particles, thus producing attrition losses. The coke is especially susceptible to attrition when the relative movement occurs while the coke is exposed to the higher temperatures of regeneration. The free falling coke is also susceptible to agglomeration of granules when containing water, dirt and/or other solids.
A microwave desorption approach which addresses the attrition problem is set forth in U.S. Pat. No. 4,322,394 to Mezey et al. The Mezey et al patent discloses a process for thermal regeneration of adsorbents wherein the adsorbent is heated dielectrically to desorb the sorbates, after which a small quantity of purge gas may be applied to flush the sorbates. The microwave heating is applied while the adsorbent is within the adsorber reactor chamber. This avoids the attrition of the adsorbent that arises from transporting the adsorbent to another apparatus for regeneration. However, desorbing the adsorbent within the adsorber reactor chamber requires that the adsorber be taken off-line for the entire process. This down time includes not only time for heating and desorbing the adsorbent but also time for cooling the system back to operating temperature. The cool down time can be quite long due to the large mass of the adsorbent and the typically massive adsorber vessel which has been be heated by conduction from the adsorbent. Consequently, the adsorber reactor chamber would be unavailable for its primary function for a significant time. Furthermore, the Mezey et al device would require significant modifications to the adsorber reactor chamber to accommodate the internal microwave heating and to provide additional valving for the separate purge gas flow.
Thus, it would be advantageous to have a desorption system which does not require a lengthy shut down of the adsorber and handles the adsorbent with minimal attrition. A non-continuous or batch system is such an approach which is particularly attractive for low throughput requirements. This approach comprises transferring the adsorbent from the adsorber vessel to a separate container and exposing the container to microwave energy in order to heat the adsorbent to the necessary temperature. The adsorber can be provided with clean adsorbent during this process and thus be kept on-line. And while a certain amount of handling of the adsorbent is required, it is minimal with no relative movement during heating. However, known batch systems have long cycle times because of slow, inefficient means for handling the adsorbent. Furthermore, the heated adsorbent is typically left in the container to cool while another, cold container is used for the next batch. Although only the adsorbent is directly heated by the microwaves, the container is heated by conduction from the adsorbent. And since the container usually holds more heat than the adsorbent, allowing the container to cool each time represents substantial lost heat and wasted energy.
Accordingly, there is a need for an on-site batch system for microwave desorption of adsorbents which has an efficient means for handling the adsorbent and retains heat in the adsorbent container between batches.